Inflow and Infiltration (I&I) Summary
What is I&I?
I&I is clean storm and/or groundwater that enters the sewer system through cracked pipes, leaky manholes, or improperly connected storm drains, down spouts and sump pumps. Most inflow comes from storm water and most infiltration comes from groundwater.
Inflow occurs when rainwater is misdirected into the sanitary sewer system instead of storm sewers. 
Examples are: roof leaders, yard and area drains, manhole covers, and cross connections from storm drains. The remedy for inflow is to remove improper connections to the sanitary sewer system.
Infiltration occurs when ground water seeps into the sanitary sewer system through cracks or leaks in sewer pipes. The cracks or leaks may be caused by age related deterioration, loose joints, damage or root infiltration. The remedy for infiltration is repairing or replacing the leaking infrastructure.
What Problems Do I&I cause?
I&I are a cause of sanitary sewer overflows and backups that release raw sewage into the environment and homes. In addition, excess storm and ground water entering the sanitary sewer system through I&I results in increased wastewater treatment costs, which are passed on to the ratepayers.
How Can Overloaded Sanitary Sewers Cause Flooding In Your Home?
The water in an overloaded sewer flows at a higher level than normal. If the home has sanitary fixtures or floor drains that are below the higher overload level, water can flow backwards through the sanitary sewer lines into the basement or other low-lying areas of the home. Section 710.1 of the Uniform Plumbing Code requires “drainage piping serving fixtures which have flood level rims located below the elevation of the next upstream manhole cover of the public or private sewer serving such drainage piping shall be protected from backflow of sewage by installing an approved type backwater valve." The diagram below illustrates where a backflow prevention valve may be required. 